1. Field of the Invention
This invention relates to vertical image scaling, more particularly for vertical image scaling using scan-line video processors.
2. Background of the Invention
As display technology continues to develop, the ability to change the size and aspect ratio of a video image has increased. New lines or picture elements (pixels) of the image are created or lines and pixels are eliminated to scale an image larger or smaller. The processing power and speed available to display systems makes these tasks virtually transparent to the viewer.
While scaling in both the horizontal and vertical dimensions is possible with just about all display technologies, it is used frequently in display devices with spatial light modulators. Spatial light modulators are devices which form images by controlling individual cells, typically one cell on the modulator array per pixel in the final image.
Each modulator cell can be individually controlled, which in turn controls the appearance of the pixel in the displayed image. This individual control allows the data for each pixel to be manipulated. Such manipulations can include generating new pixels (scaling the image larger) or eliminating pixels (making the image smaller).
Scaling in the vertical dimension can be complicated, since the incoming video data is in line format. It typically requires comparison and calculations between the current line, the previous line and the next line of video data. Because of the nature of the incoming video in rasterized format, the previous and current line must be stored until the next line becomes available. Storage of these lines requires memory, which raises the cost of the final display system.
Currently, vertical scaling implementations include extra memories. Each memory must be able to store many lines of video data. For example in a typical NTSC 3:4 upscale, 360 lines must be converted to 480 lines, a 120 line difference. This means that 120 lines of video data must be provided via interpolation, which will require storage of at least 120 lines of data. Memories do not come in 120 line sizes, so a standard field memory, capable of storing an entire field of data must by used.
In a typical scaling algorithm, there are six field memories. One memory is for the red field, one is for the green field, and the third for the blue field before the interpolation processing, for vertical upscaling. The other three field memories are for down scaling after the interpolation processing. This results in an expensive system.
Therefore, some method of vertical scaling is needed that allows a display system to perform vertical scaling without adding more memory to the system, thereby reducing the system's cost.